So, do you have SecuritySuite, then? It's a pretty decent program (I use it), but I would still get AVG Free, though, as it takes up fewer resources.
As for CCleaner...it is, as the full name implies, a crap cleaner. It will clean out your temp and Temporary Internet Files, as well as other junk. Keep in mind that freeing up space will only be a small improvement. More RAM and/or a better CPU would help you out a bit more, but I think you would be better off investing in a new computer. You're lucky that old thing is even running anymore.
EDIT: Looks like soybean pretty covered it already. Heh.